| Summary: | The effect of increasing the calcium content of a hay-grain diet fed to postparturient dairy cattle was studied. Mineral apparent digestibilities, serum concentrations and apparent mineral retentions were monitored while the animals were under the stress of peak lactation.
Nine Holstein and seven Ayrshire cows were randomly assigned to the control (0.7% Ca) or the Ca-treatment (1.5% Ca) based on calving order. The diets otherwise contained adequate nutrients. Each animal was fed to appetite for 80 days with no difference (p > . 05) occurring between treatments in intake when expessed as a percentage of body weight. Daily milk production (4% FCM) was higher (p < .05) as was the average body weight (p<.01) for the control animals reflecting the disproportionate number of young animals on the calcium treatment.
After a minimum of 60 days on trial, 5 cows from each treatment were exposed to a 5 day digestibility collection period. No change in organic matter or nitrogen apparent digestibility occurred (p>.05) but there were higher (p<.05) levels of calcium and iron, increased (p<.01) levels of copper, and lower (p<.05) zinc and molybdenum apparent digestibilities for animals on the Ca-treatment. Fecal pH was higher (p<.05) in the calcium treated cows indicating a buffering effect occurred as a result of the addition of the calcium carbonate. No change (p>.05) was evident in the secretion of minerals into the milk but urinary phosphorus excretion was significantly
higher (p<.05) in the control group.
Milk progesterone was analyzed to correspond blood samples (average of 14 per animal) to specific regions of the estrus cycle. Serum phosphorus, iron, copper and zinc varied with reproductive cycling as phosphorus dropped (p<.01) at the onset of regular estrus while the other minerals fluctuated with the cycle (copper and zinc (p< .05), iron (p<.01)). In the serum of Ca-treated animals, calcium and zinc concentrations were higher (p<.01), copper increased (p<.05), and phosphorus was lower (p<.01) than the levels for the control animals. Breed effects were apparent as both phosphorus and copper were higher (p< .01) in the serum of Ayrshires than of Holsteins. Plasma glucose concentrations proved not to be different (p>.05) between treatments.
Calcium supplementation of the diet allowed the animals to go from a negative to a positive calcium balance (p<.01). It also increased (p<.05) the amount of phosphorus apparent retention. In all, 6 essential minerals had altered apparent digestibilities and/or serum concentrations with possible long term effects on animal metabolism === Land and Food Systems, Faculty of === Graduate
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